Anaerobic digestion for treatment of stillage from cellulosic bioethanol production.

Thermophilic anaerobic digestion of stillage from a cellulosic ethanol process that uses sugarcane bagasse as feedstock was investigated. A biochemical methane potential (BMP) of 200 ml CH4 at STP (g VS)(-1) was obtained. The whole stillage was separated into two fractions: a fraction retained on 0.5 mm screen called residue and a fraction passing through 0.5 mm screen called filtrate. About 70% of total methane yield of stillage was produced from the filtrate. The filtrate was anaerobically digested in a 15 L semi-continuously fed digester operated for 91 days at HRTs of 21 and 14 days and organic loading rate (OLR) of 1.85 and 2.39 g COD L(-1) d(-1). The methane yield from the stillage from the digester was about 90% of the yield from the BMP assays. The influent soluble COD (sCOD) was reduced from between 35.4 and 38.8 g COD (L(-1)) to between 7.5 and 8 g COD (L(-1)).

Comparison of non-agitated and agitated batch, thermophilic anaerobic digestion of sugarbeet tailings.

Sugar beet tailings were anaerobically digested at non-agitated and agitated conditions in identical thermophilic batch reactors. The average methane yield in the agitated digester was only 74% of that in the non-agitated digester. Ninety percent of the ultimate methane yield was produced in approximately 5 days in the non-agitated digester whereas it took 12 days in agitated digester. Even upon using an active inoculum from non-agitated digester the methane rate and yield was low in the agitated digester. On the other hand when the poorly performing inoculum from the agitated digester was transferred to the non-agitated digester, its activity was immediately enhanced. The non-agitated digester harbored a diverse microbial community with phylotypes Methanoculleus and Methanosarcina being dominant methanogens. Methanosaeta was the only methanogen detected in the agitated digester. It also contained a hydrogen-producing bacterial phylotype Petrotoga in high proportion which was not detected in the other digester.

Biochar derived from anaerobically digested sugar beet tailings: characterization and phosphate removal potential.

Two biochars were produced from anaerobically digested and undigested sugar beet tailings through slow-pyrolysis at 600°C. The digested sugar beet tailing biochar (DSTC) and raw sugar beet tailing biochar (STC) yields were around 45.5% and 36.3% of initial dry weight, respectively. Compared to STC, DSTC had similar pH and surface functional groups, but higher surface area, and its surface was less negatively charged. SEM-EDS and XRD analyses showed that colloidal and nano-sized periclase (MgO) was presented on the surface of DSTC. Laboratory adsorption experiments were conducted to assess the phosphate removal ability of the two biochars, an activated carbon (AC), and three Fe-modified biochar/AC adsorbents. The DSTC showed the highest phosphate removal ability with a removal rate around 73%. Our results suggest that anaerobically digested sugar beet tailings can be used as feedstock materials to produce high quality biochars, which could be used as adsorbents to reclaim phosphate.

Removal of phosphate from aqueous solution by biochar derived from anaerobically digested sugar beet tailings.

Biochar converted from agricultural residues or other carbon-rich wastes may provide new methods and materials for environmental management, particularly with respect to carbon sequestration and contaminant remediation. In this study, laboratory experiments were conducted to investigate the removal of phosphate from aqueous solution by biochar derived from anaerobically digested sugar beet tailings (DSTC). Batch adsorption kinetic and equilibrium isotherm experiments and post-adsorption characterizations using SEM-EDS, XRD, and FTIR suggested that colloidal and nano-sized MgO (periclase) particles on the biochar surface were the main adsorption sites for aqueous phosphate. Batch adsorption experiments also showed that both initial solution pH and coexisting anions could affect the adsorption of phosphate onto the DSTC biochar. Of the mathematical models used to describe the adsorption kinetics of phosphate removal by the biochar, the Ritchie N_th-order (N=1.14) model showed the best fit. Two heterogeneous isotherm models (Freundlich and Langmuir-Freundlich) fitted the experimental isotherm of phosphate adsorption onto the biochar better than the Langmuir adsorption model. Our results suggest that biochar converted from anaerobically digested sugar beet tailings is a promising alternative adsorbent, which can be used to reclaim phosphate from water or reduce phosphate leaching from fertilized soils. In addition, there is no need to regenerate the exhausted biochar because the phosphate-laden biochar contains abundance of valuable nutrients, which may be used as a slow-release fertilizer to enhance soil fertility and to sequester carbon.

Single-stage, batch, leach-bed, thermophilic anaerobic digestion of spent sugar beet pulp.

Spent sugar beet pulp as received was digested in a single-stage, batch, unmixed, leach-bed, laboratory scale thermophilic anaerobic digester. Biogasification of each 0.450 kg (wet weight) batch of spent pulp was initiated by inoculating with anaerobically digested liquor from previous run. The average methane yield was 0.336 m3 CH4 at STP (kgVS)(-1), the maximum methane production rate was 0.087 m3 CH4 at STP (kgVS)(-1)d(-1), average lag time to initiate methanogenesis was only 0.44 days and time required to achieve 95% methane yield was 8 days. The pH in the digesters ranged between 8.0 and 9.5. High rates of methane generation were sustained even at high pH values. The equivalent organic loading rate in the batch digesters was 4 kgCODm(-3)d(-1). The digestion process used here offers significant improvements over one-stage and two-stage systems reported in the literature with comparable performance as it is a single-stage system where the feedstock does not require size reduction, and mixing is not required in the digester.